Actuator for scanning detecting light

ABSTRACT

Provided is an actuator for scanning detecting light, comprising an optical element for emitting detecting light, a moveable part supporting the optical element, a sheet spring having a fixed end and a moveable end supporting the moveable part; and drive means for driving the moveable part so as to scan the detecting light. Thus, a spring-mass system is formed in which the moveable part retaining the optical device acts as the mass, and the first order resonant frequency of the system may be selected so as to be higher than the operating frequency (scanning frequency). A bearing for a sliding part is not required, and the resistance loss can be thereby eliminated. These factors contribute to a favorable responsiveness. Also, by properly designing the sheet spring, a lighter and more compact design is enabled than would be possible with the conventional arrangement. A plurality of drive force generating units disposed on either side of the optical element in such a manner that the combined force of the drive force produced by the drive force generating units acts substantially onto the gravitational center of the moveable part. Thus, the drive efficiency can be improved while saving energy and achieving a high level of responsiveness.

TECHNICAL FIELD

The present invention relates to an actuator for scanning a detectinglight beam which is suitable for use in a scanning device for scanning adetecting light beam such as a laser radar.

BACKGROUND OF THE INVENTION

Conventionally, various forms of scanning devices for scanning adetecting light beam are known, such as scan type laser radars, laserscanners, laser printers, laser markers and object monitoring devices.Among such devices, actuators for scanning a detecting light beam usedin scan type laser radars for preventing a vehicle crash include thoseimpinging a light beam onto a point on a polygonal mirror which isrotated by a motor and using the light beam reflected by the polygonmirror as a detecting light beam (FIG. 21), and those using a singlemoveable mirror turned or swung by a motor to reflect a light beamimpinged thereonto from a laser light source to scan the light beam byreflection as a detecting light beam (FIG. 22) (see Japanese patent laidopen publication No. 03-175390 and Japanese patent laid open publicationNo. 07-92270).

The polygon mirror type device illustrated in FIG. 21 includes a polygonmirror 31 which is rotatively driven by an electric motor 32, and afixed reflective mirror 34 which directs a laser beam LB emitted from alaser diode 33 onto a point on the polygon mirror 31 so that thereflected laser beam may be scanned as the different reflective surfacesof the polygon mirror pass this point.

Such a polygon mirror type device is capable of a high speed scanning,but is both high is cost and large in size because a bearing is requiredfor a sliding part that rotatably supports the mirror as well as anelectric motor for swinging or rotating the mirror.

The single mirror type device illustrated in FIG. 22 includes a singlemoveable mirror 35 which is cyclically swung by an electric motor 36.Laser light emitted from a laser diode 37 is impinged upon the moveablemirror 35 to scan the laser beam LB reflected by the moveable mirror. Inthis case, the moveable mirror is typically swung in a cyclic manner byusing a cam driven by a motor.

Such a single mirror type device is suitable for compact design, andcosts less than a polygon mirror type device, but the need for a bearingfor rotatably supporting the mirror and an electric motor for drivingthe mirror thereof prevents a reduction in cost. In particular, the needto rotate a single mirror prevents an increase in the scanning speed.and the need to swing a single, mirror prevents achievement of bothcompact design and high frequency drive because of the problemsassociated with inertia and drive torque.

BRIEF SUMMARY OF THE INVENTION

To eliminate such problems, and provide an economical and compactactuator for scanning a laser beam capable of a high speed scanning, theactuator of the present invention comprises an optical element foremitting detecting light; a moveable part supporting the opticalelement; a sheet spring having a fixed end and a moveable end supportingthe moveable part; and drive means for driving the moveable part so asto scan the detecting light.

According to this arrangement, a spring-mass system is formed in whichthe moveable part retaining the optical device acts as the mass, and thefirst order resonant frequency of the system may be selected higher thanthe operating frequency (scanning frequency), A bearing for a slidingpart is not required. and the resistance loss can be thereby eliminated.This contributes to a favorable responsiveness. The optical element foremitting detecting light is not limited to devices for emitting suchlight by themselves, but may also consist of any device for changing thelight path of the detecting light emitted from detecting light emittingmeans to a desired direction.

The drive means may be provided with a plurality of drive forcegenerating units disposed on either side of the optical clement in sucha manner that the combined force of the drive force produced by thedrive force generating units acts substantially onto the gravitationalcenter of the optical element and moveable part. Thus. undesirablebehaviors resulting from an imbalance in moments can be avoided, and thedrive efficiency can be improved while saving energy and achieving ahigh level of responsiveness.

When the drive means consists of an electromagnetic force generatingunit, an electromagnetic coil which is a relatively light part of theelectromagnetic force generating unit may be provided on the moveablepart so that the mass of the moveable part may be minimized.

The optical element may comprise a mirror for reflecting detecting lightemitted from laser light emitting means. The mirror may consist of asingle mirror and a reflective surface thereof may be swung through aswinging motion of the sheet spring so that the scanning of thedetecting light can be accomplished with a simple structure.

The optical element may comprise a prism for refracting detecting lightemitted from detecting light emitting means. In this case, the incidentand exit angles of the detecting light into and out of the opticalelement can be freely selected by appropriately designing the shape ofthe prism, and this contributes to the increase in the freedom in thelayout and compact design of the actuator for scanning detecting light.

A similar effect can be obtained even when the optical element comprisesa hologram element for reflecting detecting light emitted from detectinglight emitting means,

If the optical element comprises a detecting light emitting device,detecting light can be emitted directly from the moveable part, and thepart surrounding the movable part can be made highly compact becausethere is no need for detecting light emitting means to be providedoutside the moveable part.

If the sheet spring is connected to a fixed part via a flexible circuitboard including a circuit for supplying electric current to theelectromagnetic coil, the flexible circuit board provides a dampingaction to the sheet spring.

If the sheet spring is provided with a laminated structure including anelectrically insulating layer and an electrically conductive layerserving as. a circuit for supplying electric current to theelectromagnetic coil, the circuit for supplying electric current to theelectromagnetic coil can be formed at the same time as forming the sheetspring, and the wiring work is thereby simplified.

Also, by affixing 3 viscoelastic sheet or other vibration controlmaterial to a part of the sheet spring demonstrating a relatively highstrain at the time of resonance, the resonance property can be favorablycontrolled at low cost and without substantially increasing the mass ofthe system.

The drive means may consist of an electromagnetic force generating unit,and the sheet spring may comprise a plurality of sheet spring membersdisposed one next to another in a major plane of the sheet springmembers with the electromagnetic force generating unit disposed betweenthe sheet spring members. in this case, by arranging the sheet spring sothat the electromagnetic force generating device acts substantially uponthe gravitational center of the moveable part, and the drive force isapplied substantially to the gravitational center of the moveable part,It is possible to prevent undesirable behaviors due to the imbalance inmoments from occurring. For instance, the number of component parts andthe mass of the core can be reduced and a more compact and light-weightdesign is made possible as compared to the arrangement in which a pairof electromagnetic force generating devices are arranged above and belowthe single sheet spring member in a symmetric manner.

If each of the sheet spring members has a width which gets narrower fromthe fixed end to the moveable end, the stress can be distributedsubstantially uniformly over the sheet spring, and the space foraccommodating the electromagnetic force generating unit can be favorablyensured.

If the electromagnetic force generating unit comprises anelectromagnetic coil attached to the moveable part while the coilreceives a supply of electric current via a circuit partly formed by thesheet spring members, the need for an extra wiring arrangement for theelectromagnetic coil of the moveable part can be eliminated. Therefore,any adverse effect such a wiring arrangement may have on the springproperty can be avoided while the number of component parts can bereduced, and the durability of the wiring arrangement can be improved.

If the electromagnetic force generating unit comprises a yoke attachedto the fixed part, and the yoke includes a C-shaped member which isfolded onto itself to define a gap for receiving the electromagneticcoil, the manufacturing process can be simplified.

Preferably, the electromagnetic coil is provided with an annular shape,and the yoke is attached to the fixed part so as to extend along thedirection of movement of the movable part and partly fitted into theelectromagnetic coil, the fixed part being provided with a guide partfor guiding the yoke when fitting the yoke into the electromagnetic coilalong the direction of movement of the moveable part and attaching theyoke to the fixed part. This arrangement simplifies the assembling workfor the yoke.

BRIEF DESCRIPTION OF THE DRAWINGS

Now the present invention is described in the following with referenceto the appended drawings, in which;

FIG. 1 is a general block diagram of the scan type laser radar unit 1for a vehicle crash prevention system embodying the present invention;

FIG. 2 is a schematic perspective front view showing an essential partof the scan unit 1 a;

FIG. 3 is a schematic perspective rear view showing an essential part ofthe scan unit 1 a;

FIG. 4 is an exploded perspective view showing an essential part of thescan unit 1 a;

FIG. 5 is a vertical sectional view showing the moveable part;

FIG. 6 is a fragmentary perspective view of the sheet spring and anassociated part using a flexible printed circuit board;

FIG. 7 is a diagram showing the relationship of the scanning frequencyto the fist order resonant frequency;

FIG. 8 is a vertical sectional view of the moveable part given as amodification of the first embodiment;

FIG. 9 is a fragmentary sectional view of the sheet spring given as amodification of the first embodiment;

FIG. 10 is a cross sectional view of the sheet spring given as amodification of the first embodiment;

FIG. 11 is a view similar to FIG. 9 showing a modification of the firstembodiment;

FIG. 12 is a diagram showing an embodiment using a prism as the opticalelement;

FIG. 13 is a diagram showing an embodiment using a hologram as theoptical element;

FIG. 14 is a diagram showing an embodiment using a laser emitting deviceas the optical element;

FIG. 15 is a schematic perspective front view showing an essential partof the scan unit 21 a of a scan type laser radar unit 21 for a vehiclecrash prevention system given as a second embodiment of the presentinvention;

FIG. 16 is a schematic perspective rear view showing an essential partof the scan unit 21 a;

FIG. 17 is a plan view showing an essential pan of the scan unit 21 a;

FIG. 18 is a cross sectional view taken along line XVIII—XVIII of FIG.17;

FIG. 19 is a perspective view showing the structure of the arcuate yoke28 of the scan unit 21 a;

FIG. 20 is a view similar to FIG. 17 showing the mode of assembling: thearcuate yoke 28 of the scan unit 21 a;

FIG. 21 is a schematic perspective view showing a conventional polygonmirror type laser actuator; and

FIG. 22 is a schematic perspective view showing a conventional singlemirror type laser actuator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now the present invention is described in the following in more detailin terms of a concrete embodiment with reference to the appendeddrawings.

FIG. 1 is a block diagram of a scan type laser radar unit for a vehiclecrash prevention system embodying the present invention. The scan typelaser radar unit 1 is mounted, for instance, in a front part of avehicle. The scan type laser radar unit 1 is incorporated with a scanunit 1 a consisting of a laser actuator, a scan unit control circuit 1 bfor controlling the scan unit 1 a, a laser diode 1 c serving as a laserlight emitting means for the scan unit 1 a, and a light emitting elementlighting circuit 1 d for controlling the laser diode 1 e. The laserlight beam from the laser diode 1 c is directed outward as a laser lightbeam scanned by the scan unit 1 a, and any reflected detecting lightbeam, for instance, reflected by an object ahead of the vehicle isreceived by a photodiode 1 f via a condenser lens 1 e.

The signal detected by the photodiode 1 f is amplified by an amplifyingcircuit 1 g and is then forwarded to a time measuring circuit 1 h. Theoutput signal of the time measuring circuit 1 h is forwarded to ameasurement direction computing circuit 1 i. The measurement directioncomputing circuit 1 i is connected to a scan unit control circuit 1 b, alight emitting device lighting circuit 1 d, an amplifying circuit 1 kfor a smear detection sensor 1 j, a power source circuit 1 m and aninterface circuit 1 n. The interface circuit 1 n allows the measurementdirection computing circuit 1 i to exchange signals with other controlunits such as alarm displaying means and alarm sound emitting unit.

FIG. 2 is a schematic perspective front view of an essential part of thescan unit 1 a, and FIGS. 3 and 4 are schematic perspective rear view andexploded perspective view of the same, respectively.

Referring to these drawings, a rectangular post 3 stands upright on aplate-shaped base 2 which is adapted to be attached to a casing of thescan type laser radar unit 1, and supports a base end of a sheet spring4 having a major surface extending along the axial line of therectangular post 3. A free end of the sheet spring 4 remote from therectangular post 3 fixedly carries a moveable part consisting of amirror holder 5. The mirror holder 5 retains a mirror 6 extendingperpendicularly with respect to the major surface of the sheet spring 4.

The mirror 6 reflects the laser light from the laser diode 1 c to theoutside as shown in FIG. 1, and may consist of glass, plastic or lightmetallic material such as aluminum. The surface (mirror surface) of themirror 6 is provided with a reflective layer, for instance formed bydepositing aluminum, having a smooth surface The surface of thereflective layer is coated by a protective layer consisting of SiO₂ orother thin film for the protection against corrosion and oxidization.

The mirror holder 5 fixedly carries thereon a pair of electromagneticcoils 7 a and 7 b forming a part of a drive means consisting of anelectromagnetic force generating unit at upper and lower parts thereof(as seen in the drawing), respectively, by using a bonding agent. Thetwo electromagnetic coils 7 a and 7 b are disposed symmetric about thecenter of the reflective surface of the mirror 6. as well as about thegravitational center G of the moveable part which includes the mirror 6,mirror holder 5 and electromagnetic coils 7 a and 7 b as shown in FIG.5.

A pair of arcuate yokes 8 passing through the two electromagnetic coils7 a and 7 b at upper and lower parts, respectively, and associated yokes9 each having a recessed part are integrally attached to yoke brackets10 by threaded bolts each at either end thereof, and the yoke brackets10 are in turn attached, by threaded bolts, to a yoke mounting part 11standing upright from the base 2. These yokes 8 and 9 may be formed bystamp forming soft magnetic member consisting of such materials as pureiron.

A magnet 12 is fixedly attached to the recessed part of each of thearcuate yokes 9 opposite to the corresponding arcuate yoke 8. Therefore,a magnetic flux extends between each of the magnets 12 and thecorresponding arcuate yoke 8, and the corresponding electromagnetic coil7 a or 7 b moves in the direction to cut the magnetic flux as electriccurrent is supplied to the electromagnetic coil 7 a or 7 b. Each of theelectromagnetic coils 7 a and 7 b in this case consists of approximately100 turns of copper wire wound without using a core. The material. shapeand dimensions of each of the magnets should be selected appropriatelyso as to produce a required magnetic flux and produce an adequate driveforce in cooperation with the corresponding electromagnetic coil 7 a or7 b. These components form the magnetic circuit.

The drive force produced by this magnetic circuit causes a swinging(scanning) motion to the mirror holder 5 (mirror 6) about the pivotpoint at which the sheet spring 4 is supported by the rectangular post3. Because the electromagnetic coils 7 a and 7 b are disposedsymmetrically about the gravitational center G of the moveable part asmentioned earlier, and the combined drive force of the electromagneticcoils 7 a and 7 b acts upon the gravitational center, the driveefficiency is improved, and undesirable behaviors which are otherwisecaused by the imbalance in moments can be avoided. Also, because theelectromagnetic coils 7 a and 7 b which are relatively light among thecomponents of the electromagnetic force generating unit are provided onthe moveable part, the responsiveness and power efficiency can be bothimproved.

When molding the mirror holder 5 with plastic material, the mirror 6 andelectromagnetic coils 7 a and 7 b may be insert molded at the same time.In such a case, the bonding process can be eliminated, and theproduction process can be favorably simplified. The mirror holder 5preferably consists of a light weight and high rigidity structure as amoveable part, and therefore consists of a frame structure made byinjection molding engineering plastic such as LCP (liquid crystalpolymer) and FPS (polyphenylene sulfide) filled with glass fibers so asto define empty parts as illustrated in the drawings.

The sheet spring 4 may be formed by stamp forming a thin plate membermade of beryllium copper, phosphorus copper or stainless steel. Theshape of the sheet spring 4 is selected so as to make the first orderresonant frequency of the moveable part (mirror holder 5) higher thanthe scanning frequency, and control the stress of the sheet spring 4 inuse below the fatigue limit of the material. Thus, the durability of thesheet spring against repeated stress can be ensured.

The rectangular post 3 serving as a fixed part fixedly retains the sheetspring 4, and can be made by injection molding engineering plastic suchas LCP (liquid crystal polymer) and PPS (polyphenylene sulfide) filledwith glass fibers. The sheet spring 4 may be attached to the rectangularpost 3 by using a bonding agent or by mechanically attaching it to therectangular post 3. Also, the sheet spring 4 and electromagnetic coils 7a and 7 b may be insert molded when injection molding the rectangularpost 3 and mirror holder 5. In such a cage, the bonding process can beeliminated, and the production process can be significantly simplified.

Referring to FIG. 6, a flexible printed circuit board 13 comprising anelectroconductive pattern 13 a for electrically connecting the scan unitcontrol circuit 1 b to the electromagnetic coils 7 a and 7 b is bondedentirely over the sheet spring 4. A part of the flexible printed circuitboard 13 is bent into a crank shape, and is fixedly attached to therectangular post 3 by using a bonding agent. Because the rectangularpost 3 and sheet spring 4 are attached to each other via a resilientmember (a part of the flexible printed circuit board 13), the moveablepart can be given with an appropriate damping action by selecting thematerial, thickness and shape of the flexible printed circuit board 13.Therefore, the electromagnetic coils 7 a and 7 b are not required toproduce a large braking force at each point of reversing the swingingmotion, and this contributes to the saving of the electric powerconsumption and the increase in responsiveness.

A stopper 19 made of resilient material such as rubber is attached toeach of the yoke brackets 10 by using a bonding agent or other fasteningmeans to limit the scanning angle (θ) of the moveable part in case anexcessive current is supplied to the electromagnetic coils 7 a and 7 b.When the moveable part (such as the mirror 6) has rotated more than isrequired, each of the stoppers 19 engages a part of the correspondingmirror holder 5, and limits the maximum rotational angle thereof. Thus,the moveable part is prevented from swinging excessively.

According to the scanning type laser radar unit 1 incorporated with thescan unit 1 a described above, the laser diode 1 c consists of a nearinfrared (having a wavelength in the order of 900 nm) pulse laser diode,and produces light pulses each having a duration in the order of a fewμm according to the control signal from the light emitting devicelighting circuit 1 d. The laser light from the laser diode 1 c isreflected by the mirror 6 of the scan unit 1 a, and is emitted to theoutside as a laser beam LB.

The two electromagnetic coils 7 a and 7 b receive a supply of electriccurrent corresponding to the control signal from the scan unit controlcircuit 1 b. and the mirror holder 5 (mirror 6) swings about the axialline of the rectangular post 3 according to the polarity and amplitudeof the electric current. As the angle of the reflective surface of themirror 6 changes, the laser beam LB emitted to the outside as areflected light beam undergoes a scanning or sweeping action. Theelectric current typically consists of an alternating current having afrequency in the order of 30 Hz.

An arcuate sensor 20 extends between the two yoke brackets 10 to detectthe origin and angle of the minor holder 6, and a corresponding sensingplate 20 a is attached to the lower electromagnetic coil 7 b, forinstance. The sensor 20 provides such information as the swing angle(scanning angle), angular position (absolute value), angular speed andoperating frequency. Therefore, both the distance information (which canbe computed from the state of the received pulsed laser beam) andangular information can be obtained, and it can be used for accuratelydetecting the position of a vehicle which could cause a vehicle crashThe sensor 20 and sensing plate 20 a may consist of a contact-lessoptical or magnetic encoder.

The scan unit 1 a serving as a laser actuator forms a spring-mass swing,and is given with a first order resonant frequency f₀ which is higherthan the operating frequency (scanning frequency) f_(s) to achieve afavorable responsiveness as shown in FIG. 7.

A modification of the first embodiment is described in the following.FIG. 8 is a view similar to FIG. 5 showing the modification of the firstembodiment, and the description of the remaining parts is omitted asthey are similar to those of the previous illustrated embodiment.

In this example, a film strip 4 c for damping made of polymer materialhaving a damping property is affixed to a part of the sheet spring 4which is subjected to a relatively large strain at the time of thesecond-order resonant vibration mode of the sheet spring 4. Thisposition is typically found near the free end of the sheet spring 4 orthe upper and lower positions near the electromagnetic coils 7 a and 7b, and the size, position and material of the film strip 4 c areselected so that a desired damping effect is produced. This controls theresonance peak of the second order resonance mode as indicated by thesolid line in FIG. 7, and prevents the destruction of the device due toresonant vibrations and the induction of spurious vibrations due toexternal interferences. Also, the electromagnetic coils 7 a and 7 b arenot required to produce a large braking force at each point of reversingthe swinging motion so that the energy consumption can be minimized, anda high level of responsiveness can be achieved. The imaginary line onthe right hand side of FIG. 7 shows the response that is produced whenthe film strip 4 c is not affixed.

When a vibration control member is attached to the sheet spring 4 nearthe base end thereof, the first-order resonance mode can be alsocontrolled as indicated by the imaginary line in the central part (f₀)of FIG. 7. Instead of using a film strip 4 c having a damping property,viscoelastic material in the form of gel may be applied to anappropriate part of the sheet spring. By thus selectively applying avibration control member to a suitable part, a desired frequencyresponse can be obtained without increasing the mass of the system.

Another modification of the first embodiment is described in thefollowing. FIG. 9 is a schematic perspective view of an essential partof the sheet spring given as a modification of the first embodiment, andthe remaining parts are omitted from the description as they may besimilar to those of the previous illustrated embodiment.

According to this embodiment, the sheet spring 4 of the previousembodiment is divided into upper and lower parts 4 a and 4 b, one end ofeach of the two parts 4 a and 4 b of the sheet spring is supported bythe rectangular post 3 independently, and a mirror holder 5 is supportedby the other ends of the two parts of the sheet spring 4. The sheetspring 4 may be likewise formed by stamp forming a thin plate membermade of beryllium copper, phosphorus copper or stainless steel, and canbe used as electric leads for the electromagnetic coils 7 a and 7 b.This simplifies the electric current supply circuit. The shape of thesheet spring 4 is selected so that the first order resonant frequency ofthe moveable part (the part of the mirror holder 5) is higher than thescanning frequency and the stress of the spring in operation is belowthe fatigue limit.

The sheet spring parts 4 a and 4 b should be given with a damping actionby using suitable means. For instance, a polymer film strip having adamping property may be attached to the sheet spring parts 4 a and 4 bor a viscoelastic member in the form of gel may be applied to the sheetspring parts. If a sensor signal is required to be obtained from themoveable part, a flexible printed circuit board may be used. In such acase, the flexible printed circuit board may be given with a dampingcapability, and an additional damping structure may be omitted.

FIG. 10 is a cross sectional view of yet another modification of thesheet spring given as a third embodiment of the present invention. Inthis embodiment, the sheet spring consists of a thin sheet spring 14 andforms a three layer structure by additionally comprising an electricallyinsulating layer 15 formed on the thin sheet spring 14, and anelectroconductive layer 16 for supplying electric current to theelectromagnetic coils 7 a and 7 b and a sensor signal conducting layer17 for conducting electric current for a sensor provided on the moveablepart both formed on the electrically insulating layer 15. Thisarrangement eliminates the need to form an electric current find circuitusing lead wire, and the wiring work can be simplified. If the sensingplate is formed on the moveable part as was the case with the previousembodiments, the sensor signal conducting layer 17 would not berequired.

FIG. 11 is a view similar to FIG. 9 showing yet another modification ofthe first embodiment of the present invention, and the sheet springconsists of a thin sheet spring 14. A five layer structure is formed bya pair of electrically insulating layers 15 formed on either side of thethin sheet spring 14, and an electroconductive layer 16 and a sensorsignal conducting layer 17 formed on the surface of each electricallyinsulating layer 15. This embodiment provides a similar effect as thatof the embodiment shown in FIG. 10. As compared to the three layerstructure, the number of conductive paths can be readily increased and afiner control of the electromagnetic coils 7 a and 7 b is enabled byindividually controlling the electric current supplied to each of thecoils. It is also possible to increase the number of electromagneticcoils 7 a and 7 b to four, twice that of the illustrated embodiment.Also, according to the five layer structure, because the layers arearranged symmetrically about the thin sheet spring 14, a more favorablebalancing of the spring property and damping property of the thin sheetspring is enabled as compared to the three layer structure.

The electrically insulating layers 15, electroconductive layer 16 andsensor signal conducting layer 17 may be formed by etching, pressing andpunching appropriate materials similar to those used for the flexiblecircuit board 13 in a manner suitable for mass production.

The thin sheet springs 14 used in the embodiments illustrated in FIGS.10 and 11 may be made by pressing and punching thin spring material suchas beryllium copper, phosphorus copper or stainless steel, and the shapeof the sheet spring 14 is selected so as to make the first orderresonant frequency of the moveable part (mirror holder 5) higher thanthe scanning frequency, and control the stress of the sheet spring inuse below the fatigue limit of the material.

Referring to FIG. 12, a moveable holder 15 may be provided to support aprism 16 instead of the mirror holder 5 which supported the mirror 6 inthe foregoing embodiments. This arrangement also allows the lightemitted from the laser diode 1 c to project a laser beam LB in a similarmanner as those of the foregoing embodiments via the prism 16. At thesame time, the moveable holder 15 (the prism 16) is made to undergo aswinging motion as indicated by arrow A to scan the laser beam LB. Thisembodiment also provides a similar effect. Because the swinging angle ofthe moveable holder 15 can be relatively reduced for the given scanningangle θ that is required for the laser beam LB emitted via the prism 16.Therefore, the laser beam LB can be scanned by using a relatively smallpower, and this contributes to a compact design and a lower powerconsumption.

Referring to FIG. 13, a hologram device 17 may be used instead of theprism 16 shown in FIG. 12. The parts corresponding to those of theembodiment shown in FIG. 12 are denoted with like numerals withoutrepeating the description of such parts. This arrangement also allowsthe light emitted from the laser diode 1 c to project a laser beam LB ina similar manner as those of the foregoing embodiments via the hologramdevice 17. At the same time, the moveable holder 15 (the hologram device17) is made to undergo a swinging motion as indicated by arrow A to scanthe laser beam LB. This embodiment also provides a similar effect.

Referring to FIG. 14, a light emitting device 18 such as a laser diodeserving as a laser light emitting device may be provided on the moveableholder 15, instead of the prism 16 used in the embodiment shown in FIG.12. In this case, a laser beam LB is directly emitted from the lightemitting device 18 of the moveable holder 15. At the same time, themoveable holder 15 (the light emitting device 18) is made to undergo aswinging motion as indicated by arrow A to scan the laser beam LB. Thisembodiment also provide a similar effect, and allows the partsurrounding the movable part to be made both simple and compact withoutacquiring the laser diode 1 c to be provided outside the scan unit 1 a.

FIG. 15 is a schematic perspective front view of an essential part ofthe scan unit 21 a of a scan type laser radar unit for a vehicle crashprevention system, and FIG. 16 is a schematic perspective rear view ofthe same. FIG. 17 is a plan view of the same, and FIG. 18 is a sectionalside view taken long line XVIII—XVIII of FIG. 17. As the overallstructure of the a scan type laser radar unit for a vehicle crashprevention system is similar to that of the first embodiment, theillustration and description of the same are omitted. As shown in thesedrawings, a fixed part 23 having a C-shaped side view is attached to abase 22 which is adapted to be attached to a casing of a scan type laserradar unit 1, for instance. One end (base end) of each of a pair ofupper and lower sheet spring members 24 a and 24 b is attached to thefixed part 23. Three or more sheet spring members may be used inpractice, but it is preferable to determine the number so that theelectromagnetic force generating unit may be located substantially atthe gravitational center of the moveable part. The sheet spring members24 a and 24 b are arranged in parallel to each other so that their majorsurfaces are located on a same plane. The other ends or moveable ends ofthe sheet spring members 24 a and 24 b fixedly support a mirror holder25 serving as a retaining part. The mirror holder 25 retains a singlemirror 26 Serving as an optical element in such a manner that the majorsurface of the mirror 26 extends perpendicularly to the major surface ofthe sheet spring members 24 a and 24 b. Each of the sheet spring member24 a and 24 b is provided with a width which gets progressively narrowerfrom the fixed end (base end) toward the other or moveable end thereof.Thereby, the stress of each of the sheet spring members functioning as acantilever can be distributed evenly within the sheet spring member, andthe space for accommodating the electromagnetic force generating unitcan be ensured in an efficient manner. Because the sheet spring members24 a and 24 b arranged above and below the electromagnetic forcegenerating unit in a spaced relationship, the rigidity against rollingmotion can be improved making the assembly less susceptible to externalinfluences.

The mirror 26 reflects the detecting light beam from the laser diode 21c to the outside as shown in FIG. 1, and may consist of glass, plasticor light metallic material such as aluminum. The surface (mirrorsurface) of the mirror 26 is provided with a reflective layer, forinstance formed by depositing aluminum, having a smooth surface. Thesurface of the reflective layer is coated by a protective layerconsisting of SiO₂ or other thin film for the protection againstcorrosion and oxidization.

A pair of electromagnetic coils 27 a and 27 b forming an electromagneticforce generating unit serving as a drive means are attached to themirror holder 25 in such a manner that the drive force may actsubstantially upon the gravitational G of the moveable part whichincludes the electromagnetic coils 27 a and 27 b, mirror holder 25 andmirror 26.

An arcuate yoke 28 having a lower part 28 a which is fitted into theelectromagnetic coils 27 a and 27 b is attached to the base 22. Thearcuate yoke 28 comprises a lower part 28 a and an upper part 28 b whichare spaced from each other and formed by bending a substantially annularmember provided with a bending section 28 c. A magnet 29 is fixedlyattached to the surface of the upper part 28 b facing the lower part 28a. The free ends of the upper and lower parts 28 a and 28 b remote fromthe bending section 28 c are attached to a yoke retaining part 22 a byinterposing the same and passing fasteners made of magnetic materialsuch as threaded bolts through them so as to define a magneticallyclosed structure. Therefore, a magnetic flux extends between the magnet29 and the lower part 28 a of the arcuate yoke 28, and a drive force isproduced by the electromagnetic coils 27 a and 27 b in the direction tocut across the magnetic flux as electric current is supplied to theelectromagnetic coils 27 a and 27 b.

In practice, the upper and lower parts 28 a and 28 b of the arcuate yoke28 can also be formed by joining one ends a pair of semicircularmembers, and bending the assembly into two halves. By thus forming thelower part 28 a and upper part 28 b in advance, the positional ends ofthe two parts can be accommodated to a certain extent during thefabrication process, and the assembly work can be simplified whilereducing the required number of component parts as compared to the casewhere the lower part 28 a and upper part 28 b are positioned as aseparate member when assembling the scan unit 21 a.

The drive force produced by this magnetic circuit causes a swinging(scanning) motion to the mirror bolder 25 (mirror 26) about the pivotpoint at which the sheet spring members 24 a and 24 b are supported bythe rectangular post 23. Because the drive force acts upon thegravitational center G of the moveable part as mentioned earlier, thedrive efficiency is improved, and undesirable behaviors which areotherwise caused by the imbalance in moments can be avoided. Also,because the electromagnetic coils 27 a and 27 b which are relativelylight among the components of the electromagnetic force generating unitare provided on the moveable part, the responsiveness and powerefficiency can be both improved.

As shown in FIG. 15, electrode terminals 24 c and 24 d integral with thesheet spring members 24 a and 24 b project from the back side of thefixed part 23. These electrode terminals 24 c and 24 d are connected tothe scan unit control circuit 21 b and the free ends (moveable ends) ofthe sheet spring members 24 a and 24 b are connected to theelectromagnetic coils 27 a and 27 b so that electric currant is suppliedto the electromagnetic oils 27 a and 27 b via the electrode terminals 24c and 24 d and sheet spring members 24 a and 24 b. Therefore, there isno need to attach an additional wiring member such as a flexible printedcircuit board to the sheet spring members 24 a and 24 b, and theimpairment of the responsiveness and generation of unwanted vibrationmodes due to the additional rigidity provided by such a wiring membercan be avoided. Because the electrode terminals 24 c and 24 d are verysmall and thermally highly conductive, the soldering work for theconnection can be conducted in an efficient manner.

The sheet spring members 24 a and 24 b may be formed by stamp forming oretching a thin plate member made of beryllium copper, phosphorus copperor stainless steel.

Viscoelastic material is bonded to the surfaces of the sheet springmembers 24 a and 24 b to provide an appropriate damping action to themoveable part. Therefore, a damage to the device due to resonance can beavoided, and spurious vibrations due to external disturbances can beavoided. Also, the electromagnetic coils 27 a and 27 b are not requiredto produce a large braking force at each point of reversing the swingingmotion, and this contributes to the saving of the electric powerconsumption and the increase in responsiveness.

The fixed part 23 and mirror holder 25 may be made by injection moldingengineering plastic which is both light and rigid such as LCP (liquidcrystal polymer) and PPS (polyphenylene sulfide) filled with glassfibers.

The sheet spring members 24 a and 24 b may be attached to the fixed part23 and mirror holder 25 after these parts are molded. Alternatively, thesheet spring members 24 a and 24 b may be integrally formed with thefixed part 23 and mirror holder 25 by using the sheet spring members 24a and 24 b as insert members. By so doing, the positional precision canbe improved as compared to the case where the sheet spring members 24 aand 24 b are attached to the fixed part 23 and mirror holder 25 afterthese parts are molded. It is also possible to form the two sheet springmembers as a one-piece member provided with a connecting part (not shownin the drawing) by stamp forming or etching, and remove this connectingpart after the fixed part 23 and mirror holder 25 are integrally moldedwith the sheet spring members 24 a and 24 b.

Referring to FIG. 20, the mode of assembling the scan unit 21 a isdescribed in the following. First of all, the fixed part 23, sheetspring members 24 a and 24 b and mirror holder 25 are fixedly attachedto each other, and the electromagnetic coils 27 a and 27 b are fixedlyattached to the mirror holder 25 by using a bonding agent. The fixedpart 23 is then attached to the base 22. The lower part 28 a of thearcuate yoke 28 is fitted into the electromagnetic coils 27 a and 27 b,and the yoke 28 is fixedly attached to the yoke retaining part 22 a ofthe base 22 with the yoke retaining part 22 a interposed between thelower part 28 a and upper part 28 b and the end surfaces thereofabutting the stopper surface 22 b of the bass 22. At this point, becausethe base 22 is provided with guide surfaces 22 e and 22 d for guidingthe inner circumferential surface of the arcuate yoke 28, the assemblingand positioning work of the arcuate yoke 28 is simplified.

According to the scanning type laser radar unit 21 incorporated with thescan unit 21 a described above, the laser diode 21 c consists of a nearinfrared (having a wavelength in the order of 900 nm) pulse laser diode,and produces light pulses each having a duration in the order of a few∥m according to the control signal from the light emitting devicelighting circuit 21 d. The detecting light from the laser diode 21 c isreflected by the mirror 26 of the scan unit 21 a, and is emitted to theoutside as a detecting light beam LB.

The two electromagnetic coils 27 a and 27 b receive a supply of electriccurrent corresponding to the control signal from the scan unit controlcircuit 21 b, and the mirror holder 25 (mirror 26) swings about theaxial line of the rectangular post 23 according to the polarity andamplitude of the electric current. As the angle of the reflectivesurface of the mirror 6 changes, the detecting light beam LB emitted tothe outside as a reflected light beam undergoes a scanning or sweepingaction. The electric current typically consists of an alternatingcurrent having a frequency in the order of 30 Hz. This alternatingcurrent may be PWM controlled if necessary.

The scan unit 21 a serving as a laser actuator forms a spring-massswing, and is given with a first order resonant frequency f₀ which ishigher than the operating frequency (scanning frequency) f_(s) toachieve a favorable.

Although the optical element consisted of a mirror in the foregoingembodiment, the optical element may also comprise a prism. In this case,because the swinging angle of the moveable holder can be relativelyreduced for the given scanning angle θ that is required for thedetecting light beam LB emitted via the prism, the detecting light beamLB can be scanned by using a relatively small power. This contributes toa compact design and a lower power consumption. The optical element mayalso consist of a hologram device. It is also possible that the opticalelement comprises a laser light emitting device. In such a case. adetecting light beam LB can be emitted directly from the light omittingdevice, and the part surrounding the movable part can be made highlycompact because there is no need for laser beam emitting means to beprovided outside the scan unit 1 a.

Thus, according to the present invention, a spring-mass system is formedin which the moveable part retaining the optical device acts as themass, and the first order resonant frequency of the system may beselected so as to be higher than the operating frequency (scanningfrequency) A bearing for a sliding part is not required. and theresistance loss can be thereby eliminated. These factors contribute to afavorable responsiveness. Also, by properly designing the sheet spring,such as controlling the bending stress of the sheet spring which arisesduring operation below the fatigue limit, a lighter and more compactdesign is enabled than would be possible with the conventionalarrangement using a polygon mirror. Because the moveable part isdirectly actuated while requiring fewer component parts, and thestructure is simplified, a highly low cost design is possible.

By providing a plurality of drive force generating devices symmetricallyabout the optical device, and coinciding the composite drive force withthe gravitational center of the moveable part, the drive efficiency canbe improved while saving energy and achieving a high level ofresponsiveness. An electromagnetic coil which is a relatively light partof the electromagnetic force generating unit may be provided on themoveable part so that the mass of the moveable part may be minimized.The optical clement may comprise a mirror for reflecting detecting lightemitted from laser light emitting means. The mirror may consist of asingle mirror and a reflective surface thereof may be swung through aswinging motion of the sheet spring so that the scanning of thedetecting light can be accomplished with a simple structure.

By using a prism as the optical device, the swing angle of the moveablepart can be reduced for a given swing angle of the detecting light, andthe required scanning of the detecting light can be achieved by using arelatively small drive force. By using a hologram device as the opticaldevice, a similar result can be achieved. By using a laser lightemitting device as the optical device, the detecting light can beproduced directly from the moveable part, and the need for an externallaser light emitting means can be eliminated. This allows a compactdesign of the part surrounding the moveable part.

By connecting the sheet spring to the fixed part via a flexible circuitboard for supplying electric current to the electromagnetic coils, adamping action can be produced from the flexible circuit board, and theresponsiveness can be improved. In particular, by laminating anelectrically insulating layer and an electroconductive layer forsupplying electric current to the electromagnetic coils, theelectroconductive circuit for supplying electric current can be formedat the same time as forming the sheet spring, and the assembly work suchas wiring can be simplified.

Also, by affixing a viscoelastic sheet or other vibration controlmaterial is applied to a part of the sheet spring demonstrating arelatively high strain at the time of resonance, the resonance propertycan be favorably controlled at low cost and without substantiallyincreasing the mass of the system.

If the drive means consists of an electromagnetic force generating unit,and the sheet spring comprises a plurality of sheet spring membersdisposed one next to another in a major plane of the sheet springmembers with the electromagnetic force generating unit disposed betweenthe sheet spring members in such a manner that a massive part of themoveable part is concentrated near the driving point thereof and thedrive force acts substantially upon the gravitational center of themoveable part, it is possible to prevent undesirable behaviors due tothe imbalance in monuments from occurring. Because the two sheet springmembers are spaced from each other, a rigidity against rolling motioncan be improved. Also, the number of component parts and the mass of thecore can be reduced and a more compact and light-weight design is madepossible as compared to the arrangement in which a pair ofelectromagnetic force generating devices are arranged above and belowthe single sheet spring member in a symmetric manner. Because each ofthe sheet spring members has a width which gets narrower from the fixedend to the moveable end, the stress can be distributed substantiallyuniformly over the sheet spring, and the space for accommodating theelectromagnetic force generating unit can be favorably ensured.

1. An actuator for scanning detecting light, comprising: an opticalelement for emitting detecting light; a moveable part supporting theoptical element; a sheet sprint having a fixed end and a moveable endsupporting the moveable part for a movement of the moveable part along asubstantially arcuate path centered about the fixed end of the sheetspring through a bending deflection of the sheet spring; and drive meansfor driving the moveable part along the substantially arcuate path so asto scan the detecting light, wherein the drive means is provided with aplurality of drive force generating units disposed on either side of theoptical element in such a manner that the combined force of the driveforce produced by the drive force generating units acts substantiallyonto the gravitational center of the optical element and moveable part.2. An actuator for scanning detecting light, comprising: an opticalelement for emitting detecting light; a moveable part supporting theoptical element; a sheet spring having a fixed end and a moveable endsupporting the moveable part for a movement of the moveable part along asubstantially arcuate path centered about the fixed end of the sheetspring through a bending deflection of the sheet spring, wherein avibration control member is affixed to the sheet spring at a part wherea relatively large strain is produced in a resonant vibration; and drivemeans for driving the moveable part along the substantially arcuate pathso as to scan the detecting light, wherein the drive means consists ofan electromagnetic force generating unit, and the moveable partcomprises an electromagnetic coil.
 3. An actuator for scanning detectinglight according to claim 2, wherein the optical element comprises amember selected from a group consisting of a mirror for reflectingdetecting light emitted from a laser light emitting means, a prism forrefracting detecting light emitted from a laser light emitting means, ahologram element for reflecting detecting light emitted from a laserlight emitting means, and a detecting light emitting device.
 4. Anactuator for scanning detecting light according to claim 2, wherein thesheet spring is connected to a fixed part via a flexible circuit boardincluding a circuit for supplying electric current to theelectromagnetic coil.
 5. An actuator for scanning detecting lightaccording to claim 2, wherein the sheet spring is provided with alaminated structure including an electrically insulating layer and anelectrically conductive layer serving as a circuit for supplyingelectric current to the electromagnetic coil.
 6. An actuator forscanning detecting light, comprising: an optical element for emittingdetecting light; a moveable part supporting the optical element; a sheetspring having a fixed end and a moveable end supporting the moveablepart for a movement of the moveable part along a substantially arcuatepath centered about the fixed end of the sheet spring through a bendingdeflection of the sheet spring; and drive means for driving the moveablepart along the substantially arcuate path so as to scan the detectinglight, wherein the drive means consists of an electromagnetic forcegenerating unit for driving the moveable part, and the sheet springcomprises a plurality of sheet spring members disposed one next toanother in a major plane of the sheet spring members, theelectromagnetic force generating unit being disposed between the sheetspring members.
 7. An actuator for scanning detecting light according toclaim 6, wherein the each of the sheet spring members has a width whichgets narrower from the fixed end to the moveable end.
 8. An actuator forscanning detecting light according to claim 6, wherein theelectromagnetic force generating unit comprises an electromagnetic coilattached to the moveable part, the coil receiving a supply of electriccurrent via a circuit partly formed by the sheet spring members.
 9. Anactuator for scanning detecting light according to claim 6, wherein theelectromagnetic force generating unit comprises a yoke attached to thefixed part, the yoke including a C-shaped member which is folded ontoitself to define a gap for receiving the electromagnetic coil.
 10. Anactuator for scanning detecting light according to claim 6, wherein theelectromagnetic coil is provided with an annular shape, and the yoke isattached to the fixed pert so as to extend along the direction ofmovement of the moveable part and partly fitted into the electromagneticcoil, the fixed part being provided with a guide part for guiding theyoke when fitting the yoke into the electromagnetic coil along thedirection of movement of the moveable part and attaching the yoke to thefixed part.
 11. An actuator for scanning detecting light according toclaim 6, wherein the optical element comprises a member selected from agroup consisting of: a mirror for reflecting detecting light emittedfrom a detecting light emitting means; a prism or lens for changing theoptical direction of detecting light emitted from a detecting lightemitting means; a hologram for reflecting detecting light emitted from adetecting light emitting means; and a detecting light emitting meansitself.
 12. An actuator for scanning detecting light, comprising: anoptical element for emitting detecting light; a moveable part supportingthe optical element; a sheet spring having a fixed end and a moveableend supporting the moveable part for a movement of the moveable partalong a substantially arcuate path centered about the fixed end of thesheet spring through a bending deflection of the sheet spring; and drivemeans for driving the moveable part along the substantially arcuate pathso as to scan the detecting light, the drive means including anelectromagnetic force generating unit, wherein the electromagnetic forcegenerating unit comprises a yoke extending along the arcuate path andattached to a fixed part, a magnet for supplying magnetic flux to theyoke, and an electromagnetic coil attached to the moveable part andreceiving the yoke in a central bore thereof.
 13. An actuator forscanning detecting light according to claim 12, wherein the yokeincludes a C-shaped member which is folded onto itself to define amagnetic gap for receiving a part of the electromagnetic coil.
 14. Anactuator for scanning detesting light according to claim 12, wherein thesheet spring comprises a pair of sheet spring members disposed one nextto another in a major plane of the sheet spring members, the two sheetsprings providing an electric path for the electromagnetic coil.
 15. Anactuator for scanning detecting light according to claim 12, wherein thesheet spring is provided with a laminated structure including a pair ofelectroconductive strips electrically connected to the electromagneticcoil and an insulator for providing electric insulation to theelectroconductive strips.
 16. An actuator for scanning detecting light,comprising: an optical element for emitting detecting light; a moveablepart supporting the optical element; a sheet spring having a fixed endand a moveable end supporting the moveable part for a movement of themoveable part along a substantially arcuate path centered about thefixed end of the sheet spring through a bending deflection of the sheetspring; and drive means for driving the moveable part along thesubstantially arcuate path so as to scan the detecting light, the drivemeans including an electromagnetic force generating unit, wherein theelectromagnetic force generating unit comprises a pair of yokesextending along the arcuate path in a mutually parallel relationship andattached to a fixed part, a magnet for supplying magnetic flux to theyokes, and a pair of electromagnetic coils attached to opposing ends ofthe moveable part and each receiving a corresponding one of the yokes ina central bore thereof.